Determination of apnea/hypopnea during cpap treatment

ABSTRACT

CPAP treatment apparatus is disclosed having a controllable positive airway pressure device. A sensor generates a signal representative of patient respiratory flow that is provided to a controller. The controller is operable to determine the occurrence of an apnea from a reduction in respiratory airflow below a threshold determined from long term ventilation. When an apnea or hypopnea has occurred the calculation of the threshold is suspended until the end of that event.

FIELD OF THE INVENTION

This invention relates to the administration of continuous positiveairway pressure (CPAP) treatment for partial or complete upper airwayobstruction by auto-titrating devices that adjust treatment pressure toeliminate obstructive airway events. In particular it relates to thedetection of apnea/hypopnea events.

BACKGROUND OF THE INVENTION

In the Sleep Apnea syndrome a person stops breathing during sleep.Cessation of airflow for more than 10 seconds is called an “apnea”.Apneas lead to decreased blood oxygenation and thus to disruption ofsleep. Apneas are traditionally categorized as either central, wherethere is no respiratory effort, or obstructive, where there isrespiratory effort. With some central apneas, the airway is open, andthe subject is merely not attempting to breathe. Conversely, with othercentral apneas and all obstructive apneas, the airway is closed. Theocclusion is usually at the level of the tongue or soft palate. Theairway may also be partially obstructed (i.e., narrowed or partiallypatent). This also leads to decreased ventilation (hypopnea), decreasedblood oxygenation and disturbed sleep.

The common form of treatment of these syndromes is the administration ofContinuous Positive Airway Pressure (CPAP). The procedure foradministering CPAP treatment has been well documented in both thetechnical and patent literature. An early description can be found inU.S. Pat. No. 4,944,310 (Sullivan). Briefly stated, CPAP treatment actsas a pneumatic splint of the airway by the provision of a positivepressure, usually in the range 4-20 cm H₂O. The air is supplied to theairway by a motor driven blower whose outlet passes via an air deliveryhose to a nose (or nose and/or mouth) mask sealingly engaged to apatient's face. An exhaust port is provided in the delivery tubeproximate to the mask. The mask can take the form of a nose and/or facemask or nasal prongs, pillows or cannulae.

Various techniques are known for sensing and detecting abnormalbreathing patterns indicative of obstructed breathing. U.S. Pat. No.5,245,995 (Sullivan et al.), for example, generally describes howsnoring and abnormal breathing patterns can be detected by inspirationand expiration pressure measurements made while a subject is sleeping,thereby leading to early indication of preobstructive episodes or otherforms of breathing disorder. Particularly, patterns of respiratoryparameters are monitored, and CPAP pressure is raised on the detectionof pre-defined patterns to provide increased airway pressure to,ideally, subvert the occurrence of the obstructive episodes and theother forms of breathing disorder. U.S. Pat. No. 6,502,572(Berthon-Jones et al.) generally describes a CPAP treatment apparatushaving a controllable flow generator (which is used herein as an exampleof a positive airway pressure device) operable to produce breathable gasat a treatment pressure elevated above atmosphere to a patient by adelivery tube coupled to a mask having connection with a patient'sairway. A sensor generates a signal representative of patientrespiratory flow that is provided to a controller. The controller isoperable to determine the occurrence of an apnea from a reduction inrespiratory airflow below a threshold, and if an apnea has occurred, todetermine the duration of the apnea and to cause the flow generator toincrease the treatment pressure. The '572 patent contains explicitpseudo-code for various algorithms involved in the determination of thepresence of apneas and hypopnoeas, which is included herein byreference.

BRIEF DESCRIPTION OF THE INVENTION

The present invention is directed to an improvement in theapnea/hypopnea detection algorithm of such devices as disclosed by the'572 patent to simplify it, thereby reducing its memory demands,decrease its sensitivity to noise and to remove a problem with thedetermination of long term ventilation when apneas/hypopnoeas arepresent.

The invention discloses a method for the administration of CPAPtreatment pressure comprising the steps of:

supplying breathable gas to the patient's airway at a treatmentpressure;

determining a measure of respiratory airflow; and

determining the occurrence of an apnea/hypopnea from a comparison of athreshold based upon long term ventilation with short term ventilation,wherein the determination of long term ventilation is not updated whilean apnea/hypopnea event is in progress and the apnea/hypopnea event isconsidered terminated When there is an excess of above threshold valuesof the short term ventilation;

increasing the treatment pressure during the presence of theapnea/hypopnea.

The invention further discloses a CPAP treatment apparatus forimplementing the above method comprising:

a controllable flow generator operable to produce breathable gas at apressure elevated above atmosphere;

a gas delivery tube coupled to the flow generator;

a patient mask coupled to the tube to receive said breathable gas fromthe flow generator and provide said gas, at a desired treatmentpressure, to the patient's airway;

a controller operable to receive input signals and to control operationof said flow generator and hence the treatment pressure; and

a sensor located to sense patient respiratory airflow and generate asignal input to the controller from which patient respiratory airflow isdetermined;

and wherein the controller is operable to determine the occurrence of anapnea from a comparison of a threshold based upon long term ventilationwith short term ventilation, wherein the determination of long termventilation is not updated while an apnea/hypopnea event is in progressand the apnea/hypopnea event is considered terminated when there is anexcess of above threshold values of the short term ventilation.

The recognition of the occurrence of an apnea begins by calculating anaverage respiratory airflow over a short time interval, calculating theaverage respiratory airflow over a longer time interval, and determiningwhether the average respiratory airflow over the short time interval isless than a predetermined fraction of the average respiratory airflowover the longer time interval, provided that the calculation over thelonger time interval ceases until the apnea is terminated. Forsimplicity the averages may be calculated by using an IIR filter.Occurrence and termination of an apnea or hypopnea event is determinedfrom the number of above threshold values of the short term ventilation;

In a preferred embodiment, the sensor can comprise a flow sensor, andthe controller derives respiratory airflow therefrom.

The method and apparatus can also advantageously be used in concert withthe ‘forced oscillation method’ for measuring airway patency (referredto above as European Publication No. 0 651 971 A1, U.S. Pat. No.5,704,345 whose disclosure is hereby incorporated by reference), inwhich the CPAP pressure is modulated with an amplitude of for example 1cmH₂O at 4 Hz, the induced airflow at 4 Hz is measured, the conductanceof the airway calculated by dividing the amplitude of the inducedairflow by the pressure modulation amplitude, and the additionalrequirement imposed that the treatment pressure is only increased if theconductance is greater than a threshold.

The present invention can be combined with an independent pressureincrease in response to indicators of partial upper airway obstructionsuch as snoring or changes in shape of the inspiratory flow-time curve.In this way it is possible in most subjects to achieve pre-emptivecontrol of the upper airway, with pressure increases in response topartial upper airway obstruction preventing the occurrence of closedairway apneas. In the minority of subjects in whom pre-emptive controlis not achieved, this combination will also correctly increase the CPAPpressure in response to those closed airway apneas that occur at lowCPAP pressure without prior snoring or changes in the shape of theinspiratory flow-time curve. Furthermore, the combination will avoidfalsely increasing the CPAP pressure in response to open airway apneasinduced by high pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings, in which:

FIG. 1 shows, in diagrammatic form, apparatus embodying the invention;

FIG. 2 shows an alternative arrangement of the apparatus of FIG. 1;

FIG. 3 shows the portion of the absolute value of a flow function usedto calculate ventilation.

FIG. 4 shows a typical apnea following a period of normal breathing andthe result of calculating long term averages and thresholds;

FIG. 5 shows a breathing pattern involving several apneas in succession.

FIG. 6 shows a graph of the reduction of tidal volume versus eventduration.

FIG. 7 shows a flow chart of an algorithm of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows, in diagrammatic form, CPAP apparatus in accordance withone embodiment. A mask 30, whether either a nose mask and/or a facemask, is sealingly fitted to a patient's face. Breathable gas in theform of fresh air, or oxygen enriched air, enters the mask 30 byflexible tubing 32 which, in turn, is connected with a motor driventurbine or blower 34 to which there is provided an air inlet 36. Themotor 38 for the turbine is controlled by a motor-servo unit 40 tocommence, increase or decrease the pressure of air supplied to the mask30 as CPAP treatment. The mask 30 also includes an exhaust port 42 thatis close to the junction of the tubing 32 with the mask 30.

Interposed between the mask 30 and the exhaust 42 is a linearflow-resistive element 44. In practice, the distance between mask 30 andexhaust 42, including flow resistive element 44 is very short so as tominimize dead space volume. The mask side of the flow-resistive element44 is connected by a first small bore tube 46 to a mask pressuretransducer 48 and to an input of a differential pressure transducer 50.Pressure at the other side of the flow-resistive element 44 is conveyedto the other input of the differential pressure transducer 50 by asecond small bore tube 52.

The mask pressure transducer 48 generates an electrical signal inproportion to the mask pressure, which is amplified by a first amplifier53 and passed both to a multiplexer/ADC unit 54 and to the motor-servounit 40. The function of the signal provided to the motor-servo unit 40is as a form of feedback to ensure that the actual mask static pressureis controlled to be closely approximate to the set point pressure.

The differential pressure sensed across the linear flow-resistiveelement 44 is output as an electrical signal from the differentialpressure transducer 50, and amplified by a second amplifier 56. Theoutput signal from the second amplifier 56 therefore represents ameasure of the mask airflow. The linear flow-resistive element 44 can beconstructed using a flexible-vaned iris. Alternatively, a fixed orificecan be used, in which case a linearization circuit is included in thefirst amplifier 53, or a linearization step such as table lookupincluded in the operation of controller 62.

The output signal from the second amplifier 56 is low-pass filtered bythe low-pass filter 58, typically with an upper limit of 10 Hz, in orderto remove non-respiratory noise. The second amplifier 56 output signalis also bandpassed by the bandpass filter 60, and typically in a rangeof 30-100 Hz to yield a snoring signal. The outputs from both thelow-pass filter 58 and the bandpass filter 60 are provided to thedigitizer or ADC unit 54. The digitized respiratory airflow (FLOW),snore, and mask pressure (P_(mask)) signals from ADC unit 54 are passedto a controller 62, typically constituted by a micro-processor baseddevice also provided with program memory 5 and data processing storagememory.

The controller 62 outputs a pressure request signal which is convertedto a voltage by a DAC unit 64, and passed to the motor-servo unit 40.This signal therefore represents the set point pressure P_(set)(t) to besupplied by the turbine or blower 34 to the mask 30 in theadministration of CPAP treatment. The controller 62 is programmed toperform a number of processing functions.

As an alternative to the mask pressure transducer 48, a directpressure/electrical solid state transducer (not shown) can be mountedfrom the mask with access to the space therewithin, or to the airdelivery tubing 32 proximate the point of entry to the mask 30.

Further, it may not be convenient to mount the flow transducer or linearflow resistive element 44 at or near the mask 30, nor to measure themask pressure at or near the mask. An alternative arrangement, where theflow and pressure transducers are mounted at or near the air pressuregenerator (in the embodiment being the turbine or blower 34) is shown inFIG. 2.

The pressure p_(g)(t) occurring at the pressure generator or blower 34outlet is measured by a pressure transducer 70. The flow f_(g)(t)through tubing 32 is measured with flow sensor 72 provided at the outputof the turbine or blower 34. The pressure loss along tubing 32 iscalculated in pressure loss calculation element 74 from the flow throughthe tube f_(g)(t), and a knowledge of the pressure-flow characteristicof the tubing, for example by table lookup. The pressure at the maskp_(m) is then calculated in first subtraction element 76 by subtractingthe tube pressure loss from f_(g)(t).

The pressure loss along tube 32 is then added to the desired setpressure at the mask P_(set)(t) in summation element 78 to yield thedesired instantaneous pressure at the pressure generator. Preferably,the controller of the pressure generator has a negative feedback inputfrom the pressure transducer 70, so that the desired pressure fromsummation element 78 is achieved more accurately. The flow through theexhaust 42 is calculated from the pressure at the mask (calculated infirst subtraction element 76) from the pressure-flow characteristic ofthe exhaust in exhaust flow calculation element 80, for example by tablelookup. Finally, the mask flow is calculated by subtracting the flowthrough the exhaust 42 from the flow through the tubing 32, in secondsubtraction element 82.

Calculation of Moving Average Ventilation

As depicted in FIG. 3, the instantaneous or short term ventilation iscalculated as half the θ_(ma) second moving average of the absolutevalue of a <_(RA) Hz low pass filtered respiratory airflow. Theinstantaneous ventilation is required for detection of hypopneas. Sincehypopneas can be as short as 10 seconds, the window over whichinstantaneous ventilation is calculated should be less than 10 seconds.Conversely, a single breath is typically 5 seconds. Hence instantaneousventilation should be calculated over periods of at least 5 seconds.Therefore a preferred value for θ_(ma) is 8 seconds. By using a movingwindow, instantaneous ventilation is always defined, regardless of thepoint in the respiratory cycle. Taking half the absolute value of theflow signal is identical to taking the average of the inspiratory andexpiratory flows. A preferred value for <_(RA) is 1 Hz is to preventnon-respiratory noise (snore, cardiogenic airflow) being included in themeasurement of ventilation.

The preferred long term moving average ventilation is calculated asfollows:

initialize at each mask-off to mask-on transition to 7.5 L/min for thenext 2 minutes, low pass filter the flow with time constant of 20seconds thereafter, low pass filter the flow with a time constant of 100seconds, however, cease to update the long term ventilation while anapnea/hypopnea (as determined below) is in progress.

The 100 second time constant is chosen to be long compared with theduration of a typical apnea or hypopnea (20-40 seconds), but shortcompared with genuine changes in ventilation, for example with the sleepstate (many minutes). The initialization to 7.5 L/min (a typical normalvalue), rather than zero, is so that, in the case of normal breathing,the long-term average will reach the true value more quickly. Thereduced time constant for the first 2 minutes also aids in fastersettling to the correct value.

Detection of Apnea

FIG. 4 shows a typical single 15 second apnea following a period ofnormal breathing of 24 breaths. In that figure, the dashed line showsthe long-term averaged ventilation. The dotted line is a threshold setat 25% of the longterm average flow. The solid line is a two secondmoving average flow and becomes very small during the apnea, below thethreshold.

Where there are several apneas, as shown in FIG. 5, the longterm averageventilation would be affected. Accordingly, in the present invention,the averaging of the long term ventilation is suspended upon detectionof a short term ventilation falling below the threshold, which indicatesan apnea.

Hypopnoea Detection

A hypopnoea is scored if the short term ventilation drops below 0.5times the longterm average minute ventilation (in L/sec) and hypopneasare scored if the ventilation is reduced by 50% for at least 10 seconds.Mathematically, this definition implies convolving the respiratorywaveform with a 10 second rectangular window, which will make itimpossible in principle to precisely measure the length of a hypopnea tothe nearest second. Therefore, as shown in FIG. 6, on a graph ofreduction of tidal volume versus event duration, there is a region to bescored as a hypopnea, a region to be not scored as a hypopnea, and a“don't care” region, whose shape is based on spreadsheet simulations.

Termination of Apnea/Hypopnea

To determine the termination of the apnea/hypopnea a score is maintainedas to how many data points of short term ventilation are above or belowthe threshold. Then, periodically, the score is checked. If the numberof data points below the threshold is greater than those above it isassumed the apnea is continuing, Otherwise the apnea/hypopnea event isconsidered to have ended.

Steps for Apnea/Hypopnea Detection

A logical diagram of the novel algorithm of the present invention isdepicted in FIG. 7. The steps are as follows:

1. Take the absolute value of flow and compute short-term ventilationand long term ventilation using IIR filters. Do not update long termventilation filter if apnea is detected or, hypopnea is detected formore than 10 sec.2. If short-term vent<long-term vent/4 start apnea detection, initializeduration=2 sec3. If short-term vent<long-term vent/2 start hypopnea detection,initialize duration=2 sec4. for the next 8 sec, keep marking apnea/hypopnea data as above orbelow the respective thresholds.5. at the end of 8 sec if number of data below threshold is greater thannumber of data above threshold, enter apnea and/or hypopnea state6. from now on every 4 sec period, keep marking data as above or belowthreshold (and incrementing apnea duration).7. At the end of each period, check if number of data below threshold isgreater than number of data above threshold+10.8. If true record apnea/hypopnea and enter normal state, otherwisecontinue.

More complex variants of CPAP therapy, such as bi-level CPAP therapy ortherapy in which the mask pressure is modulated within a breath, canalso be monitored and/or controlled using the methods described herein.

1. A method for the administration of CPAP treatment pressure comprisingthe steps of: supplying breathable gas to the patients airway at atreatment pressure; determining a measure of respiratory airflow; anddetermining the occurrence of an apnea/hypopnea from a comparison of athreshold based upon long term ventilation with short term ventilation,wherein the determination of long term ventilation is not updated whilean apnea/hypopnea event is in progress and the apnea/hypopnea event isconsidered terminated when there is an excess of above threshold valuesof the short term ventilation; increasing the treatment pressure duringthe presence of the apnea/hypopnea.
 2. The method of claim 1 furthercomprising the step of decreasing the treatment pressure by an amount inthe absence of an apnea/hypopnea
 3. The method of claim 2 wherein theamount is zero.
 4. The method of claim 1 wherein the occurrence of anapnea is determined by calculating an average respiratory airflow over ashort time interval, calculating the average respiratory airflow over alonger time interval, and declaring an apnea if the average respiratoryairflow over the short time interval is less than a predeterminedfraction of the average respiratory airflow over the longer timeinterval, provided that the calculation over the longer time intervalceases until the apnea is terminated.
 5. The method of claim 4 whereinthe predetermined fraction is approximately ¼.
 6. The method of claim 4wherein either the average long term ventilation or the average shortterm ventilation or both are calculated using an IIF filter.
 7. Themethod of claim 1 wherein the occurrence of an hypopnea is determined bycalculating an average respiratory airflow over a short time interval,calculating the average respiratory airflow over a longer time interval,and declaring an hypopnea if the average respiratory airflow over theshort time interval is less than a predetermined fraction of the averagerespiratory airflow over the longer time interval, provided that thecalculation over the longer time interval ceases until the hypopnea isterminated.
 8. The method of claim 6 whereby the predetermined fractionis approximately ½.
 9. The method of claim 7 wherein either the averagelong term ventilation or the average short term ventilation or both arecalculated using an IIF filter.
 10. The method of claim 1 wherein themeasure of respiratory airflow is determined using a flow sensor. 11.CPAP treatment apparatus comprising: a controllable positive airwaypressure device operable to produce breathable gas at a pressureelevated above atmosphere; a gas delivery tube coupled to the positiveairway pressure device; a patient mask coupled to the tube to receivesaid breathable gas from the positive airway pressure device and providesaid gas, at a desired treatment pressure, to the patient's airway; acontroller operable to receive input signals and to control operation ofsaid positive airway pressure device and hence the treatment pressure;and a sensor located to sense patient respiratory airflow and generate asignal input to the controller from which patient respiratory airflow isdetermined; and wherein the controller is operable to determine theoccurrence of an apnea from a comparison of a threshold based upon longterm ventilation with short term ventilation, wherein the determinationof long term ventilation is not updated while an apnea/hypopnea event isin progress and the apnea/hypopnea event is considered terminated whenthere is an excess of above threshold values of the short termventilation; and further wherein the controller is operable to increasethe treatment pressure by an amount in the presence of anapnea/hypopnea.
 12. The apparatus of claim 11 wherein the controller isfurther operable to decrease the treatment pressure by an amount in theabsence of an apnea/hypopnea.
 13. The apparatus of claim 12 wherein theamount is zero.
 14. The apparatus of claim 11 whereby the controller isfurther operable to determine the occurrence of an apnea by calculatingan average respiratory airflow over a short time interval, calculatingthe average respiratory airflow over a longer time interval, anddeclaring an apnea if the average respiratory airflow over the shorttime interval is less than a predetermined fraction of the averagerespiratory airflow over the longer time interval, provided that thecalculation over the longer time interval ceases until the apnea isterminated.
 15. The apparatus of claim 14 whereby the predeterminedfraction is approximately ¼.
 16. The apparatus of claim 14 whereineither the average long term ventilation or the average short termventilation or both are calculated using an IIF filter.
 17. Theapparatus of claim 11 whereby the controller is further operable todetermine the occurrence of an hypopnea by calculating an averagerespiratory airflow over a short time interval, calculating the averagerespiratory airflow over a longer time interval, and declaring anhypopnea if the average respiratory airflow over the short time intervalis less than a predetermined fraction of the average respiratory airflowover the longer time interval, provided that the calculation over thelonger time interval ceases until the hypopnea is terminated.
 18. Theapparatus of claim 17 whereby the predetermined fraction isapproximately ½.
 19. The apparatus of claim 14 whereby either theaverage long term ventilation or the average short term ventilation orboth are calculated using an IIF filter.
 20. The apparatus of claim 11whereby the sensor includes a flow sensor.